The present invention relates generally to turbine flow meters for use in fuel dispensing environments. More particularly, the invention relates to a turbine flow meter adapted to have enhanced accuracy across a range of fluids with varying viscosities.
Examples of turbine flow meters are shown and described in U.S. Pat. Nos. 8,381,597, 8,096,446, 6,854,342, 6,692,535, 5,831,176 and 5,689,071 (each of which is incorporated herein by reference in its respective entirety for all purposes). A turbine flow meter can be used to measure the flow rate of a liquid, which can be used to derive the volume of the liquid that has passed through the meter. In some such meters, two turbine rotors are positioned in a meter housing along the liquid flow path. As liquid passes across the rotors, they rotate. The liquid passes through the first turbine rotor and is directed into the second turbine rotor such that the second turbine rotor rotates in a direction opposite from the first turbine rotor.
During calibration of the turbine flow meter, a known volumetric flow rate of liquid is typically passed through the meter. The rotational frequency of the turbine rotors is measured at various flow rates and frequencies to arrive at a combined “Strouhal” number. The combined Strouhal number is the frequency of the rotor(s) A and B (FA and FB) divided by the volumetric flow rate (Vf) as follows:
  Strouhal  =            FA      +      FB        Vf  
The combined “Roshko” number for each of the Strouhal numbers is determined by dividing the sum of frequencies (FA+FB) by the viscosity (v) of the liquid, as follows:
  Roshko  =            FA      +      FB        v  
The Strouhal and corresponding Roshko numbers are plotted on a Roshko-Strouhal (R-S) curve and/or are stored in an array of finite points with the Strouhal numbers being plotted in one axis or an array, typically in the y-axis, and the corresponding Roshko numbers being stored in another axis or corresponding array, typically the x-axis. During operation, the R-S curve is used to determine a Strouhal number from a calculated Roshko number as discussed below.
First, the rotational frequencies of the turbine rotors are measured. As is known in the art, pick-off coils or other sensing devices, such as Hall-effect sensors, are employed in the turbine meter to detect the rotation of the turbine rotors. The sensors detect the movement of each blade on the turbine rotor and can therefore determine the frequency of rotation. Once the rotation frequencies of the turbine rotors are measured, the Roshko number can be determined according the formula for the Roshko number shown above. After the Roshko number is calculated, the corresponding Strouhal number is determined. The Strouhal number and the frequency of the turbine rotor are then used to determine the volumetric flow rate according to the rearranged Strouhal formula below:
  Vf  =            FA      +      FB        Sr  
The volumetric flow rate calculation is repeated continuously so that the volumetric flow rate of liquid flowing through the turbine meter is known at any given time. The volume of the liquid can be derived from the volumetric flow rate using time as is well known.
If the turbine flow meter is used in an application in which the liquid flow is of varying viscosities, such as in a fuel dispenser, it can be more difficult to accurately measure flow rate and volume. In known systems, a single Roshko/Strouhal curve is used by assuming a “standard” viscosity. Although the use of standard viscosity may be effective at higher flow rates, it may introduce significant error into the volumetric measurements at low flow rates (e.g., less than 2 gpm). This is because differences in viscosity result in smaller Roshko/Strouhal differences at higher flow rates rather than at lower flow rates.
Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.